This invention relates to apparatus for the individual checking of exposure to radiation, mainly X, beta and gamma rays, and also exposure to the flow of neutrons or cosmic radiation.
Exposure to certain amounts of radiation and high-energy particles is detrimental to the system on account of the destructive effect which they exert on the body cells. This true, in particular, of X, beta and gamma rays as well as, in certain cases, neutrons and cosmic rays.
As amounts of these radiations and particles cannot be directly detected by the human senses, a number of detectors have been developed in order for it to be possible to measure their intensity, to count them, or merely to confirm their presence. Among such detectors, there are Geiger counters, ionisation chambers, capacitive dosimeters, photographic films, etc.
Certain detectors measure the instantaneous value of a radiation field, for example Geiger counters and ionisation chambers, and others provide an indication as to the cumulative dose received. The latter are termed dosimeters.
The effect of radioactivity and radiation on man is cumulative, that is to say the tolerance of the body decreases as it is irradiated. If certain cells are able to regenerate, others are permanently destroyed or modified. For this reason, permitted doses do not depend merely on the intensity of radiation but also on the length of time the exposure to the radiation lasts.
Take the example of photographic film which is carried by persons who are liable to be subjected to certain types of radiation, such as the personnel in nuclear power stations. These films are periodically collected and developed and their density after development depends upon the total exposure of the user to radiation. After it has been verified that the dose received is well below what is considered to be the normal limit, the person who carried the exposed film is supplied with a new and unexposed film.
Depending upon the degree of risk involved in particular cases, the films are checked more or less frequently, the period generally varying from 24 hours to several months. This procedure, which becomes routine, is easy to put into practice within the framework of the organisation of a factory or laboratory, but it can hardly be applied to the general public.
However, the proliferation of nuclear power stations, and the reporting by the press of certain accidents during which radioactive products have escaped from controlled zones, for instance the descent of satellites carrying radioactive substances, means that many people wish to be able to check for themselves the radiation to which they may accidentally be exposed at one time or another.
As the periodic checking of all individuals can hardly be envisaged, it is desirable for everyone to be informed of the time when a period of checking is due to end, that is to say, the moment when the detector in use is due to be regenerated or changed.